1. Field of the Invention
The invention relates to a cooling device comprising Peltier elements for a thermally highly loaded optical crystal, or laser crystal, respectively, from which laser beams, in particular laser pulses, are obtained, e.g. for the laser crystal of an optical amplifier or oscillator.
2. Description of Related Art
An effective cooling of optical crystals, or laser crystals, respectively, “crystals” in short hereinafter, in laser devices is of particular importance if the crystals, e.g. titanium-sapphire crystals (commonly termed Ti:S laser crystals) are subjected to high thermal loads during operation. This is, e.g., the case if in a passively mode-locked short-pulse laser arrangement (oscillator) the crystal is utilized as an optically non-linear element, and the pump beam and the resonator beam are focussed as highly as possible in the crystal; in doing so, the crystal should have small dimensions and, for compensation thereof, a high dotation so as to keep low the material dispersion, whereby the—specific—thermal load will rise, as has been explained in the earlier application WO-98/10 494-A not previously published. There it has also been explained that cooling to below 10° C. is a problem because of the humidity condensation occurring in that instance, wherein little drops condensed on the crystal may cause the crystal to be damaged rapidly or even to be destroyed.
What is of quite particular importance is, moreover, cooling of the crystal in case of an optical amplifier, as has already been mentioned in Optics Letters Vol. 22, No. 16, Aug. 15, 1997, pp. 1256–1258, “0.2-TW laser system at 1 kHz” by Backus et al. In such an optical post-amplification of oscillator pulses, e.g., also a Ti:S laser crystal is used in which the pulses from the oscillator having an energy of some nJ are amplified to an energy in the order of 1 mJ (i.e., by the factor 106). To this end, the Ti:S amplifier crystal is “pumped” with green laser light which, e.g., has an average power of 10 to 20 W, which is a multiple of the pumping power at the laser pulse generation in the oscillator. Also by the fact that the optical amplifier is operated in pulses (the pulse frequency being, e.g., approximately 1 kHz), the pumping energy is concentrated to individual pulses which amplify the oscillator pulses. Due to the high powers occurring there, it is important to attain sufficient cooling for the crystal. Insufficient cooling of the crystal will not only result in a poor efficiency, similarly as with the oscillator, but also in an unfavorable beam profile, due to the “thermal lense” effect which also is explained in the aforementioned article by Backus et al. If the crystal is heated, the temperature gradient thus occurring in its material will lead to a refraction index gradient which will unintentionally focus or defocus the laser beam during its passage—depending on the crystal material. Good cooling of the crystal will increase the thermic conductivity of the crystal material, and the temperature coefficient of the refraction index (which causes the “thermal lense” effect) becomes smaller at the low temperatures so that a beam profile approximately corresponding to the ideal Gaussian intensity profile (over the cross-section) will be attained; moreover, the degree of efficacy will be improved. According to the article by Backus et al., liquid nitrogen is used to cool the crystal, which does make it possible to attain extraordinarily low temperatures, by which, however, a practicable embodiment of the optical amplifier is prevented for many purposes of application, in particular for mobile uses.
A somewhat different optical amplifier has been described in the article “Generation of 0.1-TW 5-fs optical pulses at a 1-kHz repetition rate” by S. Sartania et al., Optics letters Vol. 22, No. 20, Oct. 15, 1997, wherein general mention is made that a Peltier cooling device is used for cooling the amplifying crystal. Thus, the problem remains that with an intensive cooling not only condensation water will form on the crystal, but even ice, and that contaminations are present in the air which will deposit on the crystal; in operation, such ice formations and contaminations will lead to a—localized—destruction of the crystal surface by burning in.